In hydraulic and pneumatic vibration isolators, basic vibration isolation characteristics are determined by the spring or compliance volume around the operating piston and by the size of the gas or other fluid flow damping orifice or orifices out of that compliance volume. Traditional vibration isolators also have a damping chamber with a damping volume separated from the spring or compliance volume by the flow damping orifice or orifices.
The parameters that determine vibration isolation are dimensioned for the vibration isolation task predetermined for the particular isolator or group of isolators. While this optimizes the performance of the particular isolator for its predominantly conceived use, it renders the isolator, or the vibration isolation system in which it is used, vulnerable to most deviations therefrom.
For instance, load variations on vibration isolators provoke oscillations or instabilities that may take a long time to subside.
In this respect, pneumatic isolators and their load function may be viewed as a damped spring mass system excited by base motion. Transmissibility of the isolation systems increases above unity at the system resonant frequency, typically on the order of 1 to 3 Hz, is unity at 1.414 times the resonant frequency, and rolls off below unity at 40 db per decade at higher frequencies. The very low spring stiffness of the isolators, which provides the isolation, also results in a system which sags, or rises, large distances when masses are added to, or removed from, the payload. Moving masses on the payload have the same effect and result in tilting of the payload. The leveling control of the isolation system repositions and relevels the payload after several seconds. The minimum releveling time is limited by the natural frequency of the system; the lower the natural frequency, the better the isolation, but the longer the releveling time. These limitations of pneumatic isolations systems are generally not a problem for the laser/electro-optical laboratory user. For the industrial user, however, tilting of automated inspection equipment and long releveling times translate into increased cycle times and costs.
The accuracy and speed with which a conventional pneumatic isolation system compensates for, or tracks, changing loads is limited by the allowable gain of the mechanical/pneumatic servo system which controls the isolator level. The gain of the servo or leveling valve relates rate of air input to, or exhaust from, the isolators to a sensed error in payload level. When the level control gain is set very low, the air flow rate is slow and level corrections take place slowly. When the level control valve gain is set too high, rapid air inrush and exhaust cause the system to oscillate and become unstable.
Conventional pneumatic isolators of vibration isolation tables are typically dimensioned for given load situations. A series of such tables is sometimes used in a manufacturing process. The loads in such cases may be workpieces or similar objects that travel from table to table for successive manufacturing steps. Each time a load thus transfers from one table to the next in line, that one table is suddenly relieved, while the next table is suddenly loaded. This produces large table displacements in conventional systems that require continual re-leveling of tables. Reducing the time required for such table re-leveling is becoming more and more important in the context of the constant drive to speed up manufacturing processes.